The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit, therefore, must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming catalysts for application in new and existing process units.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction, thus producing a higher yield of aromatics and a lower output of fuel gas, while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates, as well as naphthas, has been disclosed. The increased sensitivity to feed sulfur of these selective catalysts also is known.
Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably, the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, regeneration of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke. This regeneration can be carried out in situ or the catalyst may be removed from the reactor where the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal.
Many hydrocarbon conversion processes, such as naphtha reforming process, employ two or more separate reactors through which a hydrocarbon feed stream passes in series. Typically, each reactor contains a bed of catalyst. The hydrocarbon feed stream passes from one reactor to the next reactor in series through conduits. In naphtha reforming, the hydrocarbon conversion reactions are endothermic, and, therefore, a heater is typically located upstream of each reactor in order to provide the necessary heat of reaction to the hydrocarbon feed stream. In addition, an indirect heat exchanger is typically located downstream of the last reactor in the series, in order to conserve energy by recovering heat from the effluent stream and transferring that heat to the feed stream upstream of the first heater.
In hydrocarbon conversion processes employing two or more reactors, arrangements for regenerating the hydrocarbon conversion catalyst in situ semi-continuously are well known. In semi-continuous regeneration, all of the reactors are periodically taken out of service and are regenerated by passing the oxygen-containing gas through the reactors in series. The oxygen-containing gas passes from one reactor to the next reactor through the heaters, heat exchangers, and conduits through which the hydrocarbon-feed stream passes when hydrocarbon conversion takes place.
The hydrocarbon feed streams of hydrocarbon conversion processes often contain sulfur. During hydrocarbon conversion, sulfur in the feed stream deposits on the surfaces of the heaters, heat exchangers and conduits which the feed stream contacts. Where iron is a component of these surfaces, the sulfur may react with the iron to form iron sulfide. Most of the deposition on and reaction with these surfaces that occurs takes place upstream of the first reactor. Consequently, in a naphtha reforming process, the indirect heat exchanger, the heater and the conduits upstream of the first reactor gradually become contaminated with sulfur.
When a catalyst is regenerated in situ in a hydrocarbon conversion process that has become contaminated with sulfur, the oxygen in the regeneration gas reacts with the sulfur to form sulfur oxide. When sulfur oxide contacts the catalyst, the catalyst may be affected in a manner such that its subsequent performance during hydrocarbon conversion is worsened. For example, depending on the particular catalyst, the regeneration conditions and the subsequent hydrocarbon conversion conditions, contacting the catalyst with sulfur oxide may displace a halogen from the catalyst or it may promote the agglomeration of catalytic metals on the catalyst. The result of either of these changes to the catalyst may adversely affect the activity and selectivity of the catalyst in promoting the desired hydrocarbon conversion reactions.
Various methods have been proposed in the prior art for regenerating sulfur-sensitive catalysts, but these methods either do not preclude the risk of contacting the catalyst with sulfur, or they require the use of additional expensive equipment. The requirement of additional expensive equipment can best be illustrated in terms of regenerating a sulfur-sensitive catalyst in a catalytic reforming process. In a catalytic reforming process, sulfur that is present in the feed stream tends to deposit on the heat exchange surfaces (tubes) of the combined feed exchanger and the charge heater. Consequently, during regeneration according to prior art processes, the sulfur-contaminated combined feed exchanger and charge heater are not used to heat the oxygen-containing streams that regenerate sulfur-sensitive catalysts, because sulfur oxide forms when oxygen contacts the tubes of the combined feed exchanger and the heater. If this sulfur oxide were passed over the sulfur-sensitive catalyst, then the select physical properties and catalytic performance of the catalyst would be damaged. Therefore, in order to avoid this risk, the combined feed exchanger and the charge heater are not employed during regeneration to heat oxygen-containing streams flowing to catalyst beds. Instead, in prior art processes, a separate regeneration exchanger and regeneration heater must be employed for this purpose. Consequently, in these prior art regeneration processes, the combined feed exchanger and the charge heater sit idle when the catalyst is being regenerated and the regeneration exchanger and regeneration heater sit idle when the catalyst is being used for reforming hydrocarbons. In short, the prior art processes require two exchangers and two heaters, because the combined feed exchanger and the charge heater cannot be employed usefully during regeneration.
Therefore, there is a need for a method of regenerating a sulfur-sensitive catalyst in a hydrocarbon conversion unit that is contaminated with sulfur in a manner that the catalyst is not contacted with sulfur.